In recent years, non-destructive sensing techniques using terahertz waves have been developed. As an application field of electromagnetic waves in this frequency band, there is a technique of performing imaging by using safe fluoroscopic devices in place of X-ray devices. Furthermore, other techniques that have been developed include a spectroscopic technique for checking the physical properties, such as a molecularly bonded state, by determining the absorption spectrum or the complex permittivity of the inside of a material, a measurement technique for checking the physical properties, such as carrier concentration, mobility, or conductivity, and a biomolecular analyzing technique. As a method of generating a terahertz wave, a method that uses a nonlinear optic crystal has been widely used. Typical examples of nonlinear optic crystals include LiNbOx (also known as LN), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. A second-order nonlinear phenomenon is used for generating a terahertz wave. Known methods for generating terahertz waves include a method of generating a difference in frequencies by inputting two beams of laser light having different frequencies, a method of generating a monochromatic terahertz pulse by an optical parametric process, and a method of generating a terahertz pulse by optical rectification using femtosecond pulse laser light.
As a process for generating a terahertz wave from a nonlinear optic crystal in this manner, electro-optic Cerenkov radiation has recently been drawing attention. As shown in FIG. 8, this is a phenomenon in which, when the propagation group velocity of laser light 100 serving as an excitation source is higher than the propagation phase velocity of a generated terahertz wave, a terahertz wave 101 is released in the form of a cone, like a shock wave. Based on a refractive-index ratio between the light and the terahertz wave within a medium (nonlinear optic crystal), a radiation angle θc is determined from the following equation.cos θc=vTHz/vg=ng/nTHZ 
In this case, vg and ng respectively denote the group velocity and the group refractive index of excitation light, and vTHz and nTHz respectively denote the phase velocity and the refractive index of a terahertz wave. There has been a report with regard to using this Cerenkov radiation phenomenon to make wavefront-tilted femtosecond laser light enter LN so as to generate a high-intensity terahertz pulse by optical rectification (see NPL 1). Furthermore, there has also been a report with regard to generating a monochromatic terahertz wave by a DFG method using a slab waveguide having a thickness that is sufficiently smaller than the wavelength of a generated terahertz wave so as to eliminate the need for a wavefront tilt (see PTL 1 and NPL 2).
Because terahertz waves are generated by traveling wave excitation in the examples in these literatures, the terahertz waves generated from different wave sources intensify each other by being phase-matched in the radiation direction, thus increasing the extraction efficiency. This radiation method is characterized in that a high-intensity terahertz wave can be generated since relatively high efficiency can be achieved when a nonlinear optic crystal is used, and that the terahertz wave band can be broadened by selecting a high-frequency side for the absorption of a terahertz region by phonon resonance, which is unique to crystals. With these techniques, the terahertz generation range can be broadened as compared with terahertz generation using photoconductors, the pulse width can be reduced in the case of terahertz pulse generation using optical rectification, and the device performance can be enhanced when the techniques are applied to, for example, terahertz time-domain spectroscopy devices.